The preparation of epoxy esters of polymerized fatty acids by conventional processes is well-known. See, for example, U.S. Pat. No. 2,940,986.
The preparation of condensation products of epoxy resins and dimer acids is described in U.S. Pat. No. 3,356,624, U.S. Pat. No. 3,408,219, and U.S. Pat. No. 3,446,762. These adducts were utilized in a traffic paint formulation wherein no curing agent is utilized; however, a considerable amount of organic solvent is required. Environmental control regulations are becoming more and more stringent, therefore, it is very desirable to prepare high solids, low-solvent coating systems. Higher solids epoxy/dimer acid adducts can be prepared by adjusting the ratio of epoxy resin to dimer acid; however, a higher than commercially desirable baking temperatures is now generally required.
The use of small amounts, i.e., less than 1%, and generally less than 0.5%, of an acid such as para-toluene sulfonic acid, to catalyze the curing of epoxy-dimer acid adducts is known; however, these systems do not have good storage stability and high baking temperatures are required to produce an acceptable coating. It would therefore be a considerable improvement if a single package system could be prepared which is not only stable, but can be cured at low temperatures, i.e. less than about 85.degree. C.
It has now been discovered that a stable, high solids, low-temperature curing epoxy baking enamel can be prepared by first preparing a high solids (80%w or above) adduct of an epoxy resin and a polymerized fatty acid; then reacting this adduct with a strong acid such as para-toluene sulfonic acid, sulfuric acid, hydrochloric acid and phosphoric acid; and finally mixing or adding an amino-plast curing resin, preferably, a high solid, liquid amino-plast resin such as an amino-formaldehyde; i.e. melamine-formaldehyde or urea-formaldehyde resin.
The resulting one-package system is stable and cures at temperatures below about 85.degree. C. to produce coatings exhibiting excellent physical properties.